Pan Pacific Symposium Conference Proceedings


TRANSFORMING FLIP CHIP INTO CSP WITH REWORKABLE WAFER-LEVEL UNDERFILL

Author: Ken Gilleo
Company: Alpha Metals
Date Published: 2/2/1999   Conference: Pan Pacific Symposium


Abstract: Many packaging specialists believe that Flip Chip is the final destination in our technology roadmap – the ultimate “package”. There is general agreement that Direct Chip Attach (DCA) provides the smallest possible footprint, the lowest profile, the highest electrical performance and also the greatest I/O capacity. Nothing can be smaller, thinner and lighter than the bare die. Yet others view the Flip Chip as only a partial solution and not even a true package. Both views are correct.

First and foremost, the electronic package must offer protection to the semiconductor device. The package must also be removable and preferably reworkable. The assembled FC remains unprotected until the underfilling step, however. Underfill adds the requisite protection and also greatly extends reliability for organic substrate systems. Underfill can be thought of as a post-packaging encapsulant material. But, while the underfill adds protection, it usually eliminates die removability and the important rework option. The underfill not only takes away one attribute while giving another, it also adds extra steps that increase manufacturing time, cost and complexity.

Many consider the “underfill problem” to be the Achilles’ heel of Flip Chip since it will ultimately hold back this otherwise powerful technology. The maligned underfill material is not the core problem however. Today’s process is! Closer examining of the underfill dilemma led us to conclude that “no flow” underfill was a move forward but an interim step. No flow products, a class of pre-dispensed underfills, still add special processes and equipment to the standard SMT assembly method but can allow an in-line process to operate. While “no flow” underfill may not be the final answer, the predispense approach is the way to go.

One simple concept is to predispense underfill onto the chip (wafer) and then solidify the material. The underfill, with flux properties, becomes an integral part of the Flip Chip. Underfilling becomes a semiconductor process just like bumping. Flip Chip assembly is returned to the realm of SMT to aptly suit the existing equipment and process infrastructure. The integral flux-underfill should be reworkable, a task made easier by the solid underfill strategy. Success with a wafer-level underfill system will make the entire process transparent to the component assembler. The Flip Chip becomes fully enabled and transformed into a true package – a simple and effective CSP. The ramifications are far reaching.

Key words: bumping, CSP, Flip Chip, flux, rework, underfill, wafer-level.



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